Electrostatic printing process



Feb. 28, 1967 K. w. RAREY 3,305,198

ELECTROSTATIC PRINTING PROCESS Filed Dec. 4, 1963 5 Sheets-Sheet l STAGE& lo

INVENTOR.

KEMN ETH W EAEEY BY bu 2L, v JAM ATTORNEYS Feb. 28, 1967 K. w. RAREY3,306,198

ELECTROSTATIC PRINTING PROCESS Filed D80. 4, 1965 5 Sheets-Sheet 2INVENTOR KENNETH w. RQREY BY W ,7, L r ATTORNEY) K. W.RAREYELECTROSTATIC PRINTING PROCESS Feb. 28, 1967 3 Sheet -Sheet 5 Filed Dec.4, 1963 INVENTOR KENNETH LU. R AREY ,o l t L CL ATTORNEYS O. .mv M

United States Fatent Ofiice 3,306,198 Patented Feb. 28, 1967 3,306,198ELECTROSTATIC PRINTING PROCESS Kenneth W. Rarey, South Holland, 111.,assignor to Continental Can Company, Inc., New York, N.Y., a corporationof New York Filed Dec. 4 1963, Ser. No. 328,036 13 Claims. (Cl. 101-426)This is a continuationin-part of application Serial No. 234,426, filedOctober 31, 1962, now abandoned.

This invention relates to electrostatic printing and more particularlyrelates to a new and useful electrostatic printing process utilizingreusable printing plates whereby copies having improved image resolutionand contrast and comparative freedom from background marking are readilyobtained.

In one present-day form of electrostatic printing, printing is done froma plate consisting of an electrically grounded conductive layer, theimage areas of which are covered with a thin dielectric layer or film.To print from the plate, the insulating layer surface is electricallycharged either positively or negatively by subjecting it to a suitableelectric discharge, such as a corona discharge or the like, and a powdercontaining microscopically sized particles of a dielectric material,having an electrical charge of opposite polarity to that of the plateinsulating layer surface, is deposited on the plate. Generally, thepowder consists of particles, called the toner, of a finely grounddielectric resin containing a pigment, the particles of which arecarried on coarser powder particles, called the carrier, which latterparticles serve both to carry the toner particles and to maintain thecorrect electrical charge on the toner.

The toner particles of the powder, when flowed onto the printing plate,remain on the raised insulating layer of the plate by electrostaticattraction to form a powder image in those areas. The carrier particles,by virtue of their individual masses, simply flow along and 01? theplate. Toner particles likewise flow from and leave the conductive areasof the printing plate.

Printed copies of the powder image are made by contacting the raisedpowder formed image with the surface of paper or other receivingsubstrate to be printed, much as in letter press printing. Transfer ofthe powder image to the receiving substrate to be printed isaccomplished by establishing an electrical charge on the receiving substrate of opposite polarity to that of the charged powder on the plateand superior to that of the insulating layer whereby the powdered imageis transferred to the receiving sheet. Once on the receiving substratesheet the image is fixed by fusing, as by heating or by solventsoftening. Another transfer technique is to roll the receiving sheetinto contact with the plate with a conductive-rubber transfer roll whileapplying a voltage to the roll on the order of about 1000 volts, therebyeffecting transfer of the image from the plate to the sheet.

To reuse the plate, it is simply necessary to recharge the insulatinglayer and apply more image forming powder.

While this type of printing has many advantages in that it is fast,economical, and dry, there is a tendency for the charged toner particlesto cling to the exposed conductive areas of the printing plate, that is,the non-image portions of the plate; and transfer of thesemis-positioned particles to the receiving substrate sheet results inundesirable background marking on the receiving sheet around thedeveloped image. While methods have been devised to minimize thiseffect, these methods also reduce the amount of image forming powderthat collects on the image areas of the plate, and pale prints result.

It is a principal object of this invention to provide an electrostaticprocess which is not only economical, fast and a completely dry processbut which furthermore produces printed copies having better contrast andsharpness with less false background markings than heretofore obtainedfrom dry electrostatic printing plate processes.

It is a specific object of this invention to provide a dry electrostaticprinting process wherein the non-image areas of the printing plateactively repel the toner particles therefrom and thus reduce thepossibility of the appearance of undesirable background markings fromtransfer of toner from the non-image areas of the plate to the receivingsheet.

It is an important object of this invention to provide an electrostaticprinting process wherein printing may be done from conductive areas ofthe printing plate, as in rotogravure printing, thereby facilitatingcleaning of the non-conductive or background areas of the plate andenabling the production of electrostatic printed copy having goodresolution.

Another important object is to provide an electrostatic printing processwhich enables printing without necessity for contact of the printingplate with the surface to be printed, thereby facilitating theproduction of distortionfree printing on corrugated and other unevensurfaces.

A further object of the invention is to provide an electrostaticprinting process whereby the transfer of the image forming material ortoner to the receiving substrate sheet is accomplished by electricallyattracting the toner to the receiving sheet while eliminating theprimary mechanism of attraction of the powdered image to the printingplate.

A still further object of the invention is to provide an electrostaticprinting process capable of effecting adhesion of relatively largeramounts of toner particles on the image pattern of the printing platethan possible heretofore, to thereby provide printed images havingbetter contrast on the receiving sheet.

Other and further objects and advantages will appear as the descriptionproceeds.

In accordance with one practice of this invention, a composite printingplate is prepared by applying a coating of insualting material to ametal plate. For example, a sheet of commercial tin plate is given anall-over coating of a resinous lacquer having an insulating solidscontent, and then baked to fix and cure the resin: normal coatingmaterials and procedures may be employed, with the end purpose of havingan all-over insulating coating of uniform thickness and characteristics.The coating is then removed from the areas from which printing is to bedone. The printing is done from an electrostatic printing plate, thenon-image areas of which are electrically charged and the image areas ofwhich are electrically conductive. The plate comprises a conductivelayer, bared surfaces of which form the image areas of the plate, and aninsulating layer over-lying the areas of the conductive surface which itis not desired to print; these insulator covered areas constitute thenon-image areas of the plate. Formation of the printable image on theplate is carried out by estab lishing an electrical charge on thenon-image insulating layer. This charge induces a charge of oppositepolarity on the surface of the conductive image areas of the plate. Thena suitable image forming material containing image forming tonerparticles having a charge of the same polarity as that of said non-imageinsulating areas is applied in such manner as to facilitate flow ofunused image material from the plate.

It has been discovered that a toner having particles of the samepolarity as that of the insulated areas of an electrostatic printingplate when applied to a plate prepared in this manner adheresexcellently to the conductive surface of the plate and will collectthereon in relatively large amounts to form dense, opaque images whichprovide prints having excellent contrast. It appears that the charge onthe insulating areas of the plate creates an electrical field whichinduces an opposite charge in the conductive areas of the plate. Thischarge is enhanced by the oppositely charged toner particles whereuponthese particles strongly adhere to the conductive areas of the plate.Since the non-image areas have a surface influencing charge of the samepolarity as that of the powder toner particles, the particles areactively repelled therefrom and little or no deposit thereof has beenfound to adhere to the insulated surface areas of the plate.Consequently, upon transferring, to the receiving substrate, the imageformed by the toner a print having good contrast and comparatively freefrom objectionable background marking results.

The toner may be applied to the plate by a variety of techniques. Apreferred procedure, from the standpoint of simplicity, is to simplycascade the dry toner particles over the plate and position the plate atan angle to the horizontal so that unused powder particlesgravitationally flow off the plate.

A two-component developer powder is preferably employed in such dryprocess, comprising carrier and toner particles. The carrier particlesare preferably spheroidal to promote flow and reduce abrasive tendenciesand are quite large as compared to the toner particles, e.g., 1 mm.compared to microns. The carrier particles and the toner particles areoppositely charged so that the toner particles collect on and arecarried by the carrier particles and clumping or agglomeration of massesof the toner particles is restricted. The toner particles are in a sizerange of from 10 to microns; the carrier particles should besufficiently massive to flow off all areas of the plate by gravity butsufiiciently small to assure deposition of the charged toner particlestherefrom onto the image areas of the plate while maintaining the tonerparticles free from undue agglomeration. Grains of sand have been foundto be excellent carrier particles.

Alternately, the metal plate may have the insulating coating appliedonly at the areas from which printing is not to be done.

The invention may also be practiced by etching a metal plate at theareas from which no printing is to be done: and then providing aninsulating filling for the etched areas only, e.g. by applying aresinous lacquer to the plate, and removing the parts of the coatingwhich overlie unetched portions of the plate thereby to provide a smoothexposed surface at a common level with portions presented by the baremetal where printing is to be done and by the insulating filling of theetched areas where no printing is to occur.

The invention will be further illustrated with reference to theaccompanying drawing wherein:

FIGURE 1 illustrates method steps and a suitable apparatus by means ofwhich the plate can be charged and the transfer on the developed imageeffected; stage a shows a printing plate including a conductive lowerlayer overlaid with an insulating layer; stage b shows the printing,

plate after a portion of the insulating layer has been removed to exposeconductive surface areas constituting the image areas of the plate;stage c illustrates the printing plate being treated by a corona source;stage d is a diagrammatic view showing the printing plate beingdeveloped by a developer mixture which includes large carrier particlesand smaller toner particles; stage e illustrates the developed printingplate with toner particles adhering to the conductive image areas; stage1 illustrates one procedure for effecting transfer of the powdered imagefrom the printing plate to a receiving substrate; stage g shows asubstrate having toner particles adhering thereto to form an imagecorresponding to the pattern determined by the conductive areas of theprinting plate; stage h shows the substrate and powdered image after thetoner particles have been fused upon the substrate;

FIGURE 2 is a schematic illustration of a suitable apparatus by means ofwhich the plate can be charged and transfer of the powder imageeffected;

FIGURES 3 to 5 are upright sections through a section of metal platebeing prepared for a modified practice of the invention;

FIGURE 6 is a perspective view of a composite plate thus prepared;

FIGURE 7 is a conventional elevation of an apparatus for charging theplates of FIGURES lb and 6;

FIGURE 8 corresponds to FIGURE 1d, and shows the development of acomposite plate after pre-charging',

FIGURE 9 is a conventional elevation of an apparatus for printing asubstrate from the pre-charged and developed plates of FIGURES 1c and 8;

FIGURES 1013 are diagrammatic views showing electrical field effects.

With reference to the accompanying drawing, it is to be understood thatthe illustrations therein are primarily schematic to facilitateunderstanding of the principles of the invention and are in nowise drawnto scale, particularly with reference to layer thicknesses, particlesizes, etc.

Referring to the sequential steps illustrated in FIGURE 1 the proceduresinvolved from the preparation of the plate to obtaining printed copiesfrom the plate are depicted in step-by-step relation in FIGURES la-lh.

FIGURE 1a discloses a printing plate 10 before preparation thereof forreception of the printing pattern thereon. The plate comprises aconductive layer 12 overlaid with an insulating layer 14 of suitableinsulator material which may be coated onto or otherwise applied overthe conductive substrate.

FIGURE 112 discloses the preparation of the plate 10 for printing byremoving portions, e.g., by mechanical or chemical etching of theinsulating layer 14 to expose surface areas 16 of the conductive layer,these exposed conductive surface areas constituting the image areas ofthe plate with the remaining surface areas 18 of the insulating layer 14constituting the non-image areas of the plate.

Thereafter, as shown at FIGURE 10 an electrical charge is established onthe insulating layer 14, e.g., by employing a conductor such as coronabar 20 charged to a potential at which corona effects appear, the chargebeing shown as positive in the illustrated embodiment. The chargedinsulating areas induce an opposite charge on the conductive layer 12,the strength of which is most pronounced on the edges of conductiveimage areas 16 of the plate.

Next, as shown at FIGURE 1d, the plate 10 is tilted at an angle tofacilitate gravity fiow and a powder 22 is dusted, by cascading orotherwise, on to the plate surface. The powder 22 is composed of carrierparticles 24 having negative charges which hold smaller particles 26having positive charges, which latter particles comprise the imageforming or toner particles. As the powder 22 strikes the plate any tonerparticles 26 released from the carrier particles 24 are repelled by thenon-image surface areas 18 of the plate and thus do not remain thereonto cause false printing later. However, these toner particles 26 areelectrically attracted to the conductive surface area 16 from whichprinted copy is to be made and consequently collect in these recessedimage areas. While the carrier particles 24 are attracted to the chargednon-image areas 18 of the plate, they are of such size and mass thatthey move along the plate by gravity (as shown by directional arrows)without being held thereon and flow into a suitable collection trough 27or the like. The attraction of toner particles to the conductive areas16 is greater than their attraction to the carrier particles 24. Thus,toner particles 26 jarred loose from the carrier particles 24 as theseparticles strike and roll over the insulating layer, are repelled by theinsulating layer and strongly attracted by the conductive surface areas16 and consequently collect thereon. Of course, the collection of tonerparticles 26 on the image areas 16 is also brought about by the greaterattraction of these image areas for the toner particles than the carrierparticles.

As shown at FIGURE 1e; by disposing the plate at an angle, the tonerparticles 26 adhere well to the conductive image areas 16 and form apowdered image thereon while the remainder of the plate remains clean.The result is a plate from which prints can be made electrostaticallywhich are exceptionally clean, i.e., free from background marking.

The procedure for effecting transfer of the powdered image from theplate 10 to a receiving substrate or sheet 28 may be effected in variousways. One of these is illustrated at FIGURE 1 wherein the plate It) isreversed relative to the corona bar from the position shown in FIGURE10, in that the insulating layer 14 is downward and the conductive layer12 on top and adjacent to corona bar 20; the substrate 28 is below theinsulating layer 14 so the particles held electrostatically on the imageareas 16 have free paths available toward the substrate. The substratecan be disposed on a suitable conductive support 30 or back electrode.The charging from the corona bar 20 then establishes a potentialdifference from the image areas 16 to the substrate. The electric fieldestablished between the conductive layer 12 of the printing plate andthe conductive support 30 serves two functions: (1) the attraction ofthe conductive image areas for the toner particles is changed to arepulsion; (2) the capability of the charged insulating regions of theplate for inducing charges which are attractive to the powder particlesis reduced; and therefore the powder particles are attracted toward thesubstrate 28 to be printed. Thereafter, the powdered image designatedgenerally by the numeral 32 is fixed and made permanent on the substrateas shown at FIGURE 1h by fusing or other means known to the art.

The schematic representation of FIGURE 2 shows a suitable high voltagesource 34 capable of supplying from about 5 to 15 thousand voltsconnected at one terminal to a backing 38 and from the other terminal ofwhich one or more long thin corona discharge wires or bars 36 extend inoverlying relation to the printing plate 10 carried on backing 38. Thehigh voltage in the wire creates an intense electrical field about thewire from which a corona discharge is emitted to electrically charge theinsulating surface 18 of the plate 10 as the plate passes thereunder inthe direction of the arrow. The backing 38 is of a conductive nature andmay constitute the shell of a drum or roller to which the plate isattached. As the printing plate 10 is passed under the wire, therelative movement being in the direction of the arrow, the non-imageinsulating areas thereof are electrically charged, thereby inducing anopposite charge on the conductive image areas 16. After charging theplate, the image powder 22 may be cascaded over the plate to form thepowdered image 32 on the conductive surface 16 and the image transferredand fixed in the manner described with reference to FIG- URE 1.

In the practice shown in FIGURES 39, a composite printing plate isprepared by etching (FIGURE 3) a metal plate 41 to a depth of about0.005 to 0.030 inch, for example by methods employed in makingelectrotypes, the recesses and depressions such as 42 being formed atthe background or non-image areas and the unetched portions 43 at theoriginal plate surfaces providing printing areas. The recesses anddepressions are then filled (FIGURE 4) with a solid insulating material44 such as a casting plastic or epoxy resin, noting that the mass canextend above the original metal surface and overlap onto image areasfrom which printing is to be done. The composite plate is then groundand polished to a smooth surface (FIGURE 5) with the metal image areasas at 45 and the insulating non-image areas as at 46 at the same level.The composite plate 40 is illustrated in FIGURE 6, with the metal areas45 forming the image to be printed, and the surrounding areas 46 beingthe non-image areas for background and provided. by the insulatingmaterial: it being understood that the thicknesses are exaggerated forclearness of illustration.

Before printing, the plate is precharged electrically, e.g. as shown inFIGURE 7. The composite plate 40 is placed on a conductive support 50,with the upper surface of the plate 40 having the image pattern of baremetal and the non-image pattern of insulating material, and with themetal base 41 in conductive contact with the support 51). A conductor 51is moved relatively along and in spaced relation to the upper platesurface, for example by supporting it on an insulating traveller 52.Several bars 51 can be employed. A source 53 establishes a potentialdifference between the conductor 51 and the meal plate 41 so that acorona discharge forms at the conductor 51.

Arrays of fine wires and of sharp metal points have been employed forthe conductor 51. Platinum and stainless steel wires of around threemils diameter have been employed: the material should be non-corrodingto avoid atmospheric deterioration over a course of time. Sewing needlesand pins have been employed in points arrays. Spacing of wires andpoints by a half inch from one another has been found satisfactory withlike spacing from the metal base 12, 41 during the precharging. Highvoltage direct current supplies of 5 to 10 kilovolts have been used ineifecting a precharging of insulation portions to a condition of activerepulsion of toner particles. Currents of 10 to microamperes have beendrawn during charging.

In FIGURE 7, the electrical charging is opposite to that of FIGURES 12,to indicate that the relative polarities of the plate 12, 41 and theinsulation 10, 46 is not critical: noting that a like change of relativepolarities of carrier and toner particles is then employed.

The conductor 51 is illustratively negative relative to the support 50,and the corona effect then causes negative charges to become establishedon the exposed surfaces of the insulation or non-image areas 45, whilethe source acts to maintain the metal body 41 of plate 40 and theexposed top surfaces 45 thereof relatively positive: this effect beingassisted by the induction of positive charges on the metal plate 41 fromand. to balance the negative charges at the surfaces of the insulationmaterial 46.

The development of the electrically charged composite plate 40 can nowbe effected as in FIGURE 8. The corona charging source 53 isdisconnected. The negative charges on the surfaces 46 of the insulationmaterial remain, with corresponding positive charges in and on the metalplate 41 and its bared top portions 45. The plate 40 can be supported inslanting position, with the charged insulation portions 46 upward, and adeveloping powder 55 is caused to cascade along the plate, as bydelivery from a hopper 56. This powder 55 can be the mixture of largerand more massive carrier particles with smaller toner particleselectrostatically carried thereby, the toner particles in FIGURE 8having a negative charge. The attractive forces between the negativetoner particles, and the positive charges at the bare metal areas 45 ofthe plate 40, cause toner particles to collect on and cover the exposedmetal as shown at 57. Conversely, there are repulsive forces between thenegative toner particles and the charges resident on the insulationareas 46, and toner does not cling thereto. The carrier particles havesuch mass that they do not come to rest on the sloping plate 40, butroll therealong and fall into a receiving trough 58.

The plate 40, with the toner accretions 57 on bare metal portions 45 andessentially absent from the insulation portions 46 can now be placed ona support or base electrode 60, FIGURE 9, for conductive contacttherewith. Insulating spacers 61 are placed upon plate 40. The substrate62, to be printed, is placed on the spacers and thus held away from theplate by a distance of, say, inch. The charge on the metal plate 41 isnow reversed in polarity, so that the bare metal portions 44 thereon nowbecome repulsive to the toner particles. This can be done by therelative movement of a conductor 63 along the substrate but spacedtherefrom, e.g. by employment of the insulating traveller 64 for theconductor 63. A high voltage source 65 is connected from its negativeterminal to the base electrode 60 and from its positive terminal to theconductor 63 so that a corona is developed at the conductor 63 andpositive charge effects are established at the surface of the substate62 opposite to that at which the printing is to be done, such positivecharges being attractive to the negative toner particles on the plate40. These particles leave the plate 40 and proceed to the substrate 62and collect thereon in a pattern determined by the shapes of the exposedmetal portions 45 from which they came. Intimate contact of the printingplate 40 and the substrate 62 is not required and the surface of thesubstrate need not be smooth.

In practice, the same source and traveller can be employed for theprecharging of FIGURE 7, and the printing transfer of FIGURE 9, byreversing the connections from source to the base and the travellingconductor.

When the source 71 is disconnected, the forces which hold the tonerparticles on the metal regions are those induced by the negative tonerparticles themselves. the electrostatic field generated from source 65for effecting transfer to the substrate acts on the one hand to causerepulsion of toner particles from the metal areas 45 employing for thispurpose the charges on the particles which have been holding them to themetal at the accretions 57; and on the other hand to attract theparticles toward the substrate 62.

In practice, it has been found that prints so made are not of uniformdensity over extended areas. The exposed surfaces of insulation regions46 are negatively charged, and there are positive charges flowing fromthe source to the metal surfaces at the bottom of the respectiverecesses. Thus each mass of insulation acts as the dielectric in acapacitor, with the several masses acting and being charged like aparallel plate capacitor. Charges of equal magnitude but oppositepolarity appear at opposite faces of the dielectric bodies, which hereare the smoothed individual masses of insulation 46; with the chargingat the exposed surface being from the corona discharge, and at thebottom surface of the respective recess by conduction from the source.Capacitors are known to exhibit relatively large electric fields in thespaces between the plates and around the edges; the field outside such acapacitoir is negligible except for this edge effect. Similarly, for thecharged printing plate, the external field is small except at the edgesof non-image areas. Only at such edges is the field large enough tosignificantly affect the motions of toner particles.

It has been found that deficiencies from this cause can be eliminated byuse of an auxiliary development electrode. This is shown in FIGURE 8 asa conductive plate 70 coextensive in area with the regions of plate 40having bare metal and insulation portions by which a pattern is to beprinted, preferably being parallel thereto and spaced therefrom so thatthe flow of the cascading particles 55 is not impeded. This spacing maybe established by local insulating supporting devices 72 at non-printingregions. A source 71 is employed to maintain the electrode 70 at thesame polarity as the charged insulation regions 46 of Hence thecomposite pattern plate 40, but at slightly less magnitude than thepotential difference between the charges on insulation regions 46 andthe metal plate 41. Thus, in the illustrated practice of FIGURES 7-9,where the development powder comprises positively charged carrierparticles and negatively charged toner particles, and the corona bars 51of FIGURE 7 have caused negative charges to be located on the insulationregions 46, the source 71 acts to make the auxiliary electrode 70negative relative to the metal plate 41. Therewith, at the regionsbetween the negatively charged insulation portions 46 and thedevelopment electrode 70, an electric field is established which acts toprevent toner from adhering to the portions 46; and conversely theelectric field between the development electrode 70 at the positivelycharged exposed metal portions 45 acts to effect toner deposition on theportions 45 regardless of their area or the induced field effects at theedges thereof. When the source 71 is disconnected, and the plate 40 withtoner 57 on the metal portions 45 thereof is withdrawn from theelectrode 70, the plate 40 can then be employed as in FIGURE 9 fortransfer of the toner to the substrate 62.

In lieu of employing a source 71 as in FIGURE 8 to establish a potentialdifference between the conductive development electrode 70 and thecomposite plate 40, the electrode 70 may have a coating of dielectricand this coating be subjected to a corona discharge, e.g., as shown forthe insulating regions 46 of plate 40 in FIGURE 7, and then employedwith this charged dielectric area at the lower surface of electrode 70in FIGURE 8 without employment of the battery 71 but preferably with theconductive elements 70 and 41 electrically connected.

The effects of an auxiliary development electrode are shown by thecomparative diagrams of FIGURES 10 to 13. In these diagrams plots orgraphs of the field effect values are superimposed above a compositeplate 40 to exemplify the field effects above exposed metal portions 45and insulation portions 46 thereof, with lines 103 showing theboundaries or edges between such portions. Each plot or graph has a zerovalue or reference base line marked 0.

The electric field intensity in the air just outside the surface of anisolated charged plate, in the direction indicated by the vector E inFIGURE 10, is essentially zero at all locations removed from the edge ofthe coating and also immediately at the edge. The field intensity isindicated by the plot or graph line -101-102-106 102-101-100 whichrepresents a zero value at points remote from the insulation area 46,and is non-zero only in narrow regions 101, 102 near and on either sideof a boundary between metal and insulation areas of the composite plate40, this boundary being projected from the plate to the graph line 100by the upright dotted line 103. A like but symmetrical condition existsat the opposite edge of the insulation area 46. The horizontal dottedlines 104, 105 in FIGURE 10 are spaced from the horizontal or zeropositions of the graph line 100 and indicate a range of fieldintensities too small in magnitude to produce a significant interactionwith a charged toner particle. The field intensity 106 above theinsulation region 46 is due to ions deposited there by exposure tocorona discharge. The non-zero field intensity 100 above the exposedmetal portions 45 of the printing plate adjacent to the coating is dueto the charge that appears near the interface, due to conduction, whenthe printing plate is charged. A charged particle would experience asignificant force due to the charge on the printing plate if broughtsufficiently near the plate in those regions where the plot of fieldintensity in FIGURE 10 falls outside the limits indicated by thehorizontal dotted lines 104, 105. If the charge of the particle is thesame as the precharge on the composite plate 40, that is, theprecharging effect resident on the insulation regions 46 of thecomposite plate 40, it will be repelled from the plate by the fieldeffect on the surface of the coating 46 and attracted toward the plate40 by the field effect at the exposed metal portions 45 adjacent to thecoating 46. The forces of repulsion are those represented by the partsof regions 102 above the non-activity reference line 104; and the forcesof attraction are those represented by the parts of region 101 below thenon-activity reference line 105. At the immediate edge of the coating,that is, at a point 107 where the boundary line 103 crosses the zeroreference line of the graph, there would be no interaction, nor willthere be interaction at regions 45 of the bare metal, such as regions108, which are substantially removed from the boundary line 103, and thefield effect at such points can be represented by the points 109 on thezero reference line. If the charge on the particle is opposite to theprecharge on the composite plate, it will be repelled from the metalregions 45 and attracted by the insulation regions 46. In each case, thefield effect at points of the insulation region 45, which points areremote from the boundary edge 103, has a value 106 in FIGURE 10, whichis not zero but is less than the edge field effects 102.

When the auxiliary electrode 70 is employed and electrically connectedto the metal plate 41 by a conductor 90, the plot of field effects is asshown in FIGURE 11. The spacing S between the composite plate 40 and theelectrode 70 must be sufficient for the development powder to beintroduced and to move for effecting the development. Assuming that thedimensions of area of the composite plate 40 with metal image regions 45and insulation non-image regions 46 are very large compared to theseparation S between the electrodes 41, 70, the electric fieldintensities may be considered as those of an infinite parallel platecapacitor. On computing and vectorially adding the field intensityvalues, the plot or graph in FIGURE 11 is obtained.

The electric field intensity between the exposed conductive metalportions of the printing plate and the development electrode is ofcourse equal to zero except near the edge of the coated portion. Aparticle with a charge of the same polarity as that on the insulationregion 46 would experience repulsion as represented by the graph portion113, when near the region 46, even when removed from the coating edge.This particle would experience little, if any, attractive force due tothe composite plate surface charge, when near the exposed conductiveportions of the plate, as represented by the graph portion 110. When theparticle is close to an exposed metal region and near a boundary line103, the zero field effect 110 thereon is modified by the presence ofthe charges on the insulation region 46, and a minor attraction 111toward the plate 40 occurs.

If the composite plate 40 has not been precharged and the charge effectson exposed metal regions 45 and on insulation regions 46 are the same,then the application of a potential difference between the electrodes41, 70, causes field effects upon a particle in the space s between theelectrodes as shown by the plot or graph lines in FIGURE 12. Assumingthe auxiliary electrode 70 negatively charged relative to the metalplate 41, the field effect on a negatively charged particle between theelectrodes is an attraction represented by graph portions 121 oppositemetal regions 45 and a lesser attraction represented by graph portion122 opposite the insulation portion 46 due to negative charges inducedon the portions 46 by the positive charges in the underlying plate 41.

When the composite plate 40 has been precharged, and the electrode 70 isat a potential V relative to the relative to the metal plate 41, thefield effects represented in FIGURES 11 and 12 are combined, asrepresented by the plot or graph lines in FIGURE 13. With the plate 41connected to the positive pole of source 71, FIGURE 8, and the auxiliarydevelopment electrode 70 connected to the negative pole, graph portions130 represent a Zero electric field effect remote from the space betweenthe electrodes 41, 70. A negative toner particle between the electrodesis subjected to a field effect 131 when above an exposed metal region 45by which the particle is attracted to the metal, and to a field effect132 when above an insulation region 46 by which it is repelled therefromby a uniform force over essentially the entire area of the region 46 andwith no greater force effect at the edges or boundaries 103 as shown at102 in FIGURE 10. Conversely, the edge effects 133 of greaterattraction, adjacent the boundaries 103, toward the metal regions 45 aregreatly reduced in result, compared to the effects 101 in FIGURE 10,noting that the field effects above most of the metal regions 45 werezero in FIGURES 10 and 11, but are dominant at 131 in FIGURE 13.

The potential drop V between the electrodes 41, 70 must be less than theprecharging effect resident on the insulation regions 46.

The effect of employing the auxiliary development electrode 70, With acharge of the same polarity as the precharging upon the insulationregions and that upon the toner particles being employed for printing,is to provide toner accretions 57 of uniform density upon the exposedmetal pattern regions 45 and to maintain the insulation pattern regions46 free of toner, and to sharply delimit the boundaries between suchregions. Therewith prints are produced in which the image area of tonerare sharply fined and of uniform depth of tone, while the background ornon-image areas are clear of toner. The printed areas may be of largeextent without loss of depth.

The process is ideally suited for the reproduction of line copy, areacopy and half tone copy and since the powdered image is formed on theconductive arears of the printing plate rather than on the insulatorcovered areas thereof, the powdered image is formed in those areas wherethe electrical fields of force are strongest.

The process is a relatively rapid one, each of the charging, dusting andprinting steps requiring less than about one second. By carrying thesteps out in rapid sequence and by beginning to copy the second printbefore the first is completed, the processing time per print can bereduced to but a few seconds to give printed images having excellentcontrast and sharpness of definition on almost any kind of receivingsubstrate, whether the receiving substrate be smooth surfaced orirregularly surfaced as by wafiling, corrugating, etc. Since printingcan be accomplished without requiring contact of the receiving substrateto be printed with the image areas of the printing plate, excellent andundistorted prints can be obtained on such uneven surfaced material ascorrugated board.

The practice of the invention is further illustrated by the examplesfollowing:

Example 1 A printing plate was prepared as in FIGURES 1a and 112 from asheet of tinplate coated with a protective lacquer by removing portionsof the lacquer to expose the tinplate on the area to be printed byscraping off the lacquer with a sharp metal stylus. The lacquer coatingwas an insulator having an average thickness on the plate of less thanabout 1 mil and comprising an oleoresin of about equal parts of a meleicacid resin ester and China-wood oil. The lacquer remaining on thetinplate was provided with an electrical charge as in FIGURE 10 bypassing the plate under a plurality of corona discharge wires from anapparatus such as that illustrated in FIGURE 2, the wire diameter being2 mils and the plate to Wire distance being about /2 to A1 of an inch.The high voltage source generated from 5000 to about 15,000 volts andproduced a corona effect around the wire, thereby establishing thecharge on the remaining insulating layer. The plate was retained on aconductive support, the electrical connections being those illustratedin FIGURE 2.

After charging, the plate and support were tilted as in FIGURE 1d at anangle to the horizontal and an image powder cascaded thereover. Theimage powder consisted of carrier particles of an Ottawa-type sandcoated with ethyl cellulose, which particles averaged about sevententhsof a millimeter in diameter and were generally spherioidal. Carried onthese carrier particles were toner particles of about microns indiameter, the toner particles comprising a styrene-methyl methacrylatepolymer in which were suspended fine particles of carbon black, thestyrene and ethylacrylate were present in about a 1:1 weight ratio.

After excess powder had cascaded off the plate so that only tonerparticles remained to form the powdered image on the conductive imagesurface areas of the plate, a sheet of corrugated board was placed onthe support and the printing plate turned up-side-down as in FIGURE 1 sothat the powdered image thereon faced the corrugated board receivingsheet. This was again run under the corona discharge wires whereupon thepowdered image was transferred to the face of the corrugated board asshown in FIGURE 1g. The powdered image deposited on the corrugated boardsurface was fixed and made permanent by heating the printed area of theboard under a photo-flood lamp.

The resultant printed corrugated board as in FIGURE 111 displayed animage which had excellent contrast and sharpness of detail with nodistortion. While a few false-printing spots occured on the face of theboard, they were widely scattered and appeared to be the result ofdefects in the insulating surface of the printing plate.

Example 2 Following generally the procedures in Example 1, printedcopies were made from a printing plate of anodized aluminum having aphotographic coating thereover. The photographic coating was exposed andthe unexposed areas of the coating overlying the anodizing coating ofthe aluminum and the anodized coating in these areas were removed toexpose the aluminum surface by treatment with a solution of lye.

Following the procedure of the preceding example excellent printedcopies were made.

Example 3 In order to prevent contact of a copy to be printed with theprinting area of the printing plate, a plate was prepared as in Example1 and the copy to be printed maintained about of an inch away from theplate while the image transferred thereto. It was found that copiescould be readily made in this manner.

In the printing methods shown, the composite plate 10 or 40 with tonerparticles on the exposed metal portions thereof, does not have thedevelopment field of FIGURES 1d and 8 present during the printing. Theonly field effect to be overcome in transferring the toner from theplate 10 or 40 to the substrate 28 or 62 is that due to the inductionfrom the charged toner particles and which has been retaining them onthe exposed metal regions of the composite plate: as distinguished fromdeveloping and printing by use of electric forces which cause the tonerparticles to form deposits upon insulation portions of the compositeplate, and wherein the pre-charge upon such portions must be overcomebefore the particles can be detached therefrom and caused to migrate tothe substrate.

Furthermore, the effects of differing thicknesses or non-homogeneitiesin the insulation regions are minimized, noting that such regionsacquire different potentials during the precharging. Thus when suchinsulation regions are employed to hold the toner for printing, theamount so held is not uniform over the area, and the printcorrespondingly has different depths of tone. With the instantprocedure, the toner is repelled from such insulation regions, and theexposed metal, with its uniform conductivity and smooth surface,collects the toner during development.

Like examples of practice have been performed as described for FIGURES 3to 13.

Following these procedures, such diverse receiving substrates as glass,corrugated board, plastic and metal have been successfully printed. Theprocess is equally useful for printing on insulators or conductors sincethe practice of the process relies on the establishment of differencesin electrical potentials between the plate and the image powder.

While coated sand granules were used as the carrier particles in thespecific examples noted hereinbefore, it is to be understood that theinvention is not limited to the utilization of any particular carrier ortoner particles in the image powder: it is only necessary that thecarrier and the toner particles be sufficiently far apart in thetriboelectric series so that the particles will assure a sufiicientcharge, the electrical attraction between them being such that the tonerparticles will be carried by the carrier without tending to agglomeratetoo greatly.

As noted hereinbefore, the size range of the toner particles ispreferably between about 10 and about 30 microns for the production ofgood printed copy. The pigment in the toner may be carbon black: andother coloring ingredients, such as vegetable dyes or other colorantswhich do not adversely affect the dielectric properties of the tonerparticles, can be used to provide a visible image.

The size of the carrier particles should not be so large as to preventdeposition of toner particles on the desired printing areas or so smallas to permit agglomeration of toner particles or retention of carrierparticles on the plate. A preferred size range is from about 0.1millimeter to about one millimeter.

For the insulator layer on the printing plate, any of the materialsnormally used for insulating can be made which maintain an electricalcharge effect for a sufficient time to permit the development at theimage areas.

While the invention has been described with respect to the polarity ofthe electrical charges, polarity differences are not critical. Thus, solong as a difference in electrical potential exists between theconductive image areas of the plate and the non-image areas of the platesuch that the charged toner particles are sufficiently attracted to theformer to adhere thereto and repelled from the latter, the invention isoperative.

I claim:

1. A method of printing from a printing plate upon a substrate, saidmethod comprising the steps of:

(a) providing a printing plate having image and nonimage areas ofdifferent electrical potentials, the image areas being electricallyconductive surfaces and the non-image areas being electricallyinsulative surfaces;

(b) charging said non-image areas for providing said non-image areaswith a predetermined polarity;

(c) developing the conductive image areas with an image-forming materialwhich is comprised of electrically charged image particles which have apolarity the same as the polarity of the charged nonirnage areas;

(d) establishing a potential difference between the conductive surfacesof said printing plate and said substrate for transferring theimage-forming material from said conductive surfaces to said substrate,the potential difference being established so that the polarity of thesubstrate is opposite to the polarity of the charged image particleswhereby the charged image particles are attracted from said conductivesurfaces to said substrate.

2. The method of claim 1 in which the image forming material alsocomprises carrier particles electrically 13 attracted by said non-imagearea and repelled by said image area, the electrical forces between thecarrier "and image particles being less than the attractive forcesbetween the image particles and the image area when the carrier andimage particles are adjacent the image area.

3. The method of claim 2 in which the carrier particles have massesgreater than the masses of the image particles, and in which the saidimage and non-image areas are positioned so that the image formingpowder moves therealong under the action of gravity whereby imageparticles are captured by the image area and the carrier particles moveover the non-image areas and are competent to separate from thenon-image areas, image particles adhering thereto.

4. The method of claim 2 in which the image area is provided by anexposed portion of a surface of a metal body and the non-image area isprovided by the exposed surface of an insulating layer overlying otherportions of said body surface.

5. The method of claim 1, including the step of presenting a developmentelectrode in spaced relation to the surface of said printing plate whichhas the image and non-image areas, and in which the image formingmaterial is brought between the printing plate and the developmentelectrode, and comprising the step of establishing a potentialdifference between the printing plate and the development electrodeduring the application of the image forming material for maintaining thedevelopment electrode at the polarity of the insulative non-image areasand at a lesser electrical potential.

6. A method of electrostatic printing which comprises providing aprinting plate having a conductive underlayer and an insulatingoverlayer, portions of said conductive underlayer being exposed andforming the image area of said plate, utilizing the remaining overlyingportions of said insulating layer to form insulative non-image areas ofsaid plate, providing said insulative non-image areas with an electricalcharge of one polarity and thereby inducing an electrical charge ofdifferent polarity on said conductive image areas, then applying to saidplate an image forming powder containing charged image forming particleshaving a polarity opposite to the polarity of the electrical charge onthe conductive image areas so as to be attracted to said conductiveimage area and repelled from said insulative non-image area,transferring the powdered image formed by said particles on saidconductive image areas to a substrate to be printed by creating anelectrical potential difference between said image area and saidsubstrate whereby said particles are repelled by said image area andattracted to said substrate, and thereafter fixing said powdered imageon said substrate.

7. A method of electrostatic printing which comprises providing aprinting plate having a conductive image surface area and anelectrically insulative non-image surface area, establishing anelectrical charge of one polarity on said insulative non-image area andthereby inducing a charge of opposite polarity on said image area, thenapplying to said plate an image powder comprising carrier particleshaving a charge of the same polarity as said image area and to which areelectrostatically adhered toner particles having a charge of ppositepolarity to that of said image area and thereby causing toner particlesto be repelled from said insulative non-image area and to separate fromcarrier particles and to collect and form a powder image on said imagearea, re moving carrier particles and excess toner particles from saidplate, and then transferring said powder image to a substrate to beprinted therewith by establishing a charge on said substrate of thepolarity opposite that of said powder image while establishing a chargeof the same polarity as said powder image on said conductive surfacearea of said plate, and thereafter fusing said powder image to saidsubstrate.

8. The method of claim 7 wherein said electrical charge is establishedin said non-image area by subjecting said plate to a corona discharge.

9. The method of claim 7 wherein said electrical charge on saidnon-image area and on said substrate are established by coronadischarge.

10. A method of printing from a metal printing plate upon a substrate,which comprises providing a printing plate having conductive image andinsulative non-image areas and applying a potential therebetween forproviding said image and non-image areas with charges of oppositeelectrical polarities, the image areas being provided by exposed metalsurfaces of the plate and the non-image areas having their surfacesinsulated from the metal of the plate, positioning a developmentelectrode parallel to and spaced from the said image and non-image areasof the printing plate, applying an electrical potential to thedevelopment electrode which is of the same polarity as and of lesserpotential than the charged non-image areas of said printing plate,bringing between the development electrode and the printing plate animage forming material comprising electrically charged image particleshaving a predetermined polarity and attracted by said image areas andrepelled by said non-image areas whereby a powder image is formed uponsaid image areas, and thereafter transferring said image formingmaterial to the substrate, by establishing a potential differencebetween said image areas and the surface of the substrate so that thepolarity of said image areas is the same as the polarity of saidparticles and so that the polarity of the substrate is opposite to thepredetermined polarity of said image particles and said particles of theimage-forming material are repelled by the image areas and received bythe substrate.

11. The method of electrostatic printing, which comprises preparing aprinting plate of conductive material having recesses in its surface atnon-image areas, depositing non-conductive insulation material in saidrecesses to fill the same and establishing a common exposed surface onsaid plate comprising conductive image areas of exposed metal andnon-conductive insulation regions constituting exposed non-image areas,electrically precharging the insulation regions at the exposed areasthereof, applying a charged non-conductive image-forming powder to saidcommon exposed surface with the particles having an electrical charge ofthe same polarity to that of the pre-charge upon the insulation regionswhereby the particles are repelled from the insulation areas and areattracted to and collect on said exposed metal areas, and thereaftertransferring the image forming particles from said plate to thesubstrate to be printed by establishing a potential difference betweenthe metal plate and the surface of the substrate such that the polarityof the metal plate is the same as the polarity of said particles.

12. The method of printing from a composite printing plate upon asubstrate comprising the steps of providing a printing plate having aconductive base layer constituting that part of the printing plate fromwhich printing is to be done and an exposed patterned insulation layerthereon with the conductive base layer exposed between the parts of theinsulation pattern, providing said base layer and insulation layer withcharges of differing electrical polarities, positioning a developmentelectrode parallel to and spaced from said composite plate, applyingparticles of a printing toner powder to the surface of the compositeplate, said particles being of opposite charge to the parts of saidcomposite plate from which printing is to be done, applying anelectrical potential to the development electrode which is of the samepolarity as the image particles whereby to facilitate the collection ofa uniform layer of the toner particles at the parts of said compositeplate from which printing is to be done and thereafter transferring thetoner particles from the composite plate to the substrate to be printedby establishing a potential difierence between the composite plate andthe substrate with the substrate at a polarity opposite to that of thetoner particles.

13. The method of claim 12, in which the toner particles'are applied asa component of a development powder having carrier particles of largermass and toner particles of lesser mass, the carrier and toner particleshaving opposite electrical charges, and in which the composite plate anddevelopment electrode are positioned at an angle to the horizontal sothat the carrier particles are caused by gravity forces to move acrossthe face of the composite plate.

References Cited by the Examiner UNITED STATES PATENTS DAVID KLEIN,Primary Examiner.

1. A METHOD OF PRINTING FROM A PRINTING PLATE UPON A SUBSTRATE, SAIDMETHOD COMPRISING THE STEPS OF: (A) PROVIDING A PRINTING PLATE HAVINGIMAGE AND NONIMAGE AREAS OF DIFFERENT ELECTRICAL POTENTIALS, THE IMAGEAREAS BEING ELECTRICALLY CONDUCTIVE SURFACES AND THE NON-IMAGE AREASBEING ELECTRICALLY INSULATIVE SURFACES; (B) CHARGING SAID NON-IMAGEAREAS FOR PROVIDING SAID NON-IMAGE AREAS WITH A PREDETERMINED POLARITY;(C) DEVELOPING THE CONDUCTIVE IMAGE AREAS WITH AN IMAGE-FORMING MATERIALWHICH IS COMPRISED OF ELECTRICALLY CHARGED IMAGE PARTICLES WHICH HAVE APOLARITY THE SAME AS THE POLARITY OF THE CHARGED NONIMAGE AREAS;